This invention relates generally to optical scanners and displays. More particularly, this invention relates to an optical raster-scanning microelectromechanical system.
Scanning micromirrors fabricated using surface-micromachining technology are known in the art. As used herein, a micromirror, a microscopic device, a micromachined device, a micromechanical device, or a microelectromechanical device refers to a device with a third dimension above a horizontal substrate that is less than approximately several milli-meters. Such devices are constructed using semiconductor processing techniques.
Scanning micromirrors have numerous advantages over traditional scanning mirrors. For example, they have smaller size, mass, and power consumption, and can be more readily integrated with actuators, electronics, light sources, lenses and other optical elements. More complete integration simplifies packaging, reducing the manufacturing cost. These factors add motivation to the development of microfabricated scanners. In addition to displays, high-speed, high-resolution micro-optical scanners have numerous additional applications in medicine, lithography, printing, data storage and data retrieval.
U.S. Pat. No. 5,867.297 (the ""297 patent) entitled xe2x80x9cApparatus and Method for Optical Scanning with an Oscillatory Microelectromechanical Systemxe2x80x9d describes early seminal work in the field of oscillatory micromirrors. The contents of the ""297 patent are expressly incorporated by reference herein.
The required system tolerances in a system of the type described in the ""297 patent are extremely high. For example, bending of torsional hinges causes system wobble, defined as rotation about an axis in the mirror plane orthogonal to the primary scan axis. In a two mirror system including a fast mirror and a slow mirror, fast mirror wobble of less than 1% of the total deflection angle will cause scan lines to overlap and seriously degrade image quality. In the slow mirror, rotational errors known as jitter, attributable to errors in following the driving signal, can induce non-uniform line spacing. It would be highly desirable to establish improved mechanical linkages to enhance mirror performance.
Large mirror diameters and rotational angles, facilitated by a tilt-up mirror design, are key to the resolution of a scanning system. Moving a large mirror quickly through a large angle requires high-force actuators and stiff springs to achieve a high resonant frequency. Mechanically, the image resolution is limited by the number of lines that the fast mirror can scan during the refresh period of the slow mirror. Optically, the resolution is given by the size, flatness and rotational angle of the mirror. Increasing the mirror diameter results in higher resolution only if the mirror is flat, or if its curvature is optically corrected. It would be highly desirable to provide a method of characterizing and correcting static mirror curvature to improve the performance of an optical raster-scanning system.
A method of operating a micromechanical scanning apparatus includes the steps of identifying a radius of curvature value for a micromechanical mirror and modifying a laser beam to compensate for the radius of curvature value. The identifying step includes the step of measuring the far-field optical beam radius of a laser beam reflected from the micromechanical mirror. The measured far-field optical beam radius is then divided by a theoretical far-field optical beam radius reflected from an ideal mirror to yield a ratio value M. An analytical expression for M is curve-fitted to experimental data for M with the focal-length as a fitting parameter. The focal-length value determined by this procedure, resulting in a good fit between the analytical curve and the experimental data is equal to half the radius of curvature of the micromechanical mirror.
The micromechanical scanning apparatus is operated by controlling the oscillatory motion of a first micromechanical mirror with a first micromechanical spring and regulating the oscillatory motion of a second micromechanical mirror with a second micromechanical spring.
The invention provides an improved optical raster-scanning micromechanical system. Mirror performance in the system is improved through the technique of characterizing and correcting static mirror curvature. Improved mechanical linkages that exploit symmetry reduce mirror wobble. A triangular control signal maximizes the linearity of the scan.